For various applications, for example as a lightweight substitute for mineral glass for producing panes for automobile and aircraft construction or as embedding compositions for optical, electronic or optoelectronic components, there is increasing interest on the market today in transparent, lightfast polyurethane compositions.
Particularly for high-quality optical applications, for example for lenses or spectacle glasses, there is generally a desire for plastics materials exhibiting high refraction coupled with low dispersion (high Abbe number).
The production of transparent polyurethane masses having a high refractive index has been described many times already. Araliphatic diisocyanates, i.e. diisocyanates whose isocyanate groups are bonded via aliphatic radicals to an aromatic system, are often used as polyisocyanate components. Owing to their aromatic structures, araliphatic diisocyanates afford polyurethanes having an increased refractive index while at the same time the aliphatic isocyanate groups guarantee the lightfastness and low yellowing tendency required for high-quality applications.
U.S. Pat. No. 4,680,369 and U.S. Pat. No. 4,689,387 describe polyurethanes/polythiourethanes suitable as lens materials for example, the production of which involves combining special sulfur-containing polyols or mercapto-functional aliphatic compounds with monomeric araliphatic diisocyanates, for example 1,3-bis(isocyanatomethyl)benzene (m-xylylene diisocyanate, m-XDI), 1,4-bis(isocyanatomethyl)benzene (p-xylylene diisocyanate, p-XDI), 1,3-bis(2-isocyanatopropan-2-yl)benzene (m-tetramethylxylylene diisocyanate, m-TMXDI) or 1,3-bis(isocyanatomethyl)-2,4,5,6-tetrachlorobenzene, in order to achieve particularly high refractive indices.
Monomeric araliphatic diisocyanates such as m- and p-XDI or m-TMXDI are also mentioned in numerous further publications, for example EP-A 0 235 743, EP-A 0 268 896, EP-A 0 271 839, EP-A 0 408 459, EP-A 0 506 315, EP-A 0 586 091 and EP-A 0 803 743, as the preferred polyisocyanate component for producing high refractivity lens materials. They serve as crosslinker components for polyols and/or polythiols and, depending on the coreactant, afford transparent plastics having high refractive indices in the range from 1.56 to 1.67 and comparatively high Abbe numbers up to 45.
A substantial disadvantage of the mentioned processes for producing high refractivity polyurethanes/polythiourethanes for optical applications is that some of the manufactured lenses do not always meet the desired standards in terms of their transparency and freedom from cloudiness. This applies especially to application in lenses and the like.
In order to satisfy the high requirements of optical applications, complex production processes for the raw materials are currently prior art. Thus, EP-A 1 908 749 describes the production of XDI, where initially the hydrochloride of the XDA is produced and this is then phosgenated under elevated pressure in order to obtain correspondingly pure grades. A direct phosgenation of the XDA would be desirable.